5,10,15,20-Tetrakis(1-methylpyridinium-4-yl)porphyrin tetraiodide tetrahydrate

The asymmetric unit of the title compound, C44H38N8 2+·4I−·4H2O, comprises two halves of non-equivalent cations of 5,10,15,20-tetrakis(1-methylpyridinium)porphyrin (with the full molecule of each completed by the application of inversion symmetry), four charge balancing iodide anions and four water molecules of crystallization (two water molecules are fully occupied and four molecules have a site occupancy of 50%). The porphyrin cations are arranged into supramolecular columns parallel to the b axis, mediated by π–π [centroid–centroid distance = 3.762 (4) Å] and C—H⋯π supramolecular interactions [C⋯centroid distance = 3.522 (7) Å, C—H⋯centroid = 128°], leading to the formation of columns parallel to the b axis. The close packing leads to the presence of a one-dimensional channel filled with partially occupied water molecules engaged in O—H⋯O and O—H⋯I hydrogen bonds


Related literature
Data collection: APEX2 (Bruker, 2006); cell refinement: APEX2; data reduction: SAINT-Plus (Bruker, 2005); program(s) used to solve structure: SHELXTL (Sheldrick, 2008); program(s) used to refine structure: SHELXTL; molecular graphics: DIAMOND (Brandenburg, 2009); software used to prepare material for publication: SHELXTL. With the increasing number of antibiotic-resistant strains of microorganisms it is imperative to find alternative treatments for microbial infections (Alves et al., 2008). Photodynamic therapy is a promising non-antibiotic approach to photoinactivate antibiotic-multi-resistant pathogenic microorganisms. The photodynamic destruction of microorganisms is based on the ability of certain photosensitizers, when activated by light, generate reactive oxygen species that are able to destroy or affect bacterial membranes (Carvalho et al., 2009). For example, studies using cationic meso-substituted porphyrins acting as photosensitizers revealed interesting results in the destruction of Gram-positive and Gram-negative bacteria (Alves et al., 2009;Banfi et al., 2006;Merchat et al., 1996). The compound 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (Gomes et al., 2011) is one of the most used photosensitizers in the photo-inactivation studies of microorganisms. Results have shown that the cationic conjugates are able to efficiently photosensitize different types of microorganisms (Gomes et al., 2011;Tomé et al., 2004;Alves et al., 2008;Yu et al., 2009). Following our on-going interest on organic crystals (Paz et al., 2002;Paz & Klinowski, 2003) and in water clusters confined in organic/hybrid matrices (Shi et al., 2008), here we wish to describe the crystal structure of the title compound.
The asymmetric unit ( Fig. 1)  4H 2 O, comprises two distinct halves of centrosymmetric tetracationic porphyrin molecules, whose charge is balanced by four iodide anions, plus four water molecules of crystallization. The porphyrin rings are planar (with deviations from planarity smaller than ca 0.16 Å) subtending angles with the substituent pyridinium rings which range from ca 56 to ca 68°. The crystal structure is rich in weak supramolecular interactions such as π-π stacking and C-H···π interactions: there are two π-π stacking interactions between adjacent pyrrole rings [Cg···Cg distances of 3.762 (4) and 4.108 (4) Å], and there is a single C35-H35···π interaction [C···Cg of 3.522 (7) Å] between a hydrogen from the pyridinium ring and a pyrrole ring (interactions not shown). These supramolecular interactions contribute to the formation of columns of porphyrin molecules which are parallel to the b axis as depicted in Fig. 2. These columnar arrangements close pack in the ac plane leading to the formation of one-dimensional channels which, due to the need of close proximity of the iodide anions with the pyridinium rings, are instead filled with disordered water molecules of crystallization engaged in hydrogen bonding interactions (Table 1 and Figs 2 and 3). We note that a sole water molecule (O1W) is located between two iodide anions (I2 and I3) and outside the aforementioned channel, deeply embedded into the hydrophobic portion of the crystal structure. The remaining three water molecules are distributed among five independent crystallographic sites which may be involved in several O-H···O hydrogen bonding interactions as depicted in Fig. 3 (see Table 1 for geometric details). It is also interesting to note that only one iodide anion (I1) is not close to water molecules, participating instead in several short contacts with the porphyrin cations, among which a C-H···Iweak hydrogen interaction (d D···A = 3.691 (6) Å) arises as the strongest and more directional one (green dashed lines in Fig. 2). I2 to I4 are also involved in such type of interactions (not shown) as summarized in Table 1. supplementary materials sup-2 Experimental Crystals of the title compound have been isolated using the synthetic procedure described in detail by Gomes et al. (2011).

Refinement
Hydrogen atoms bound to carbon and nitrogen were placed in idealized positions with C-H = 0.95 Å (aromatic) or 0.98 Å (terminal methyl groups), and N-H = 0.88 Å. These atoms were included in the final structural model in riding-motion approximation. The isotropic thermal displacement parameters for these atoms were fixed at 1.2 (for the aromatic H atoms) or 1.5 (for the terminal -CH 3 moieties) times U eq of the atom (C or N) to which they are attached.
Four water molecules of crystallization were found to be partially occupied and were included in the final structural model with fixed rates of occupancy of 50% (calculated from unrestrained refinement for the site occupancies). Hydrogen atoms associated with water molecules could not be located from difference Fourier maps and attempts to include these in calculated positions did not lead stable structural refinements. Nevertheless, the hydrogen atoms associated with these chemical entities have been included in the empirical formula of the title compound.
The structural model contains a large residual electron densities of 2.73 and -2.79 e . Å -3 located at 0.91 and 0.82 Å from the I4 atom, respectively. Attempts to include these peaks as a disordered iodide anion did not lead to sensible structural refinements.

Figures
Fig . 1. Molecular units comprising the asymmetric unit of the title compound. The porphyrin molecules have been completed (through inversion symmetry) for clarity. Displacement ellipsoids are drawn at the 50% probability level and the atomic labeling is provided for all non-hydrogen atoms comprising the asymmetric unit. Hydrogen atoms are represented as small spheres with arbitrary radius.   Table 1.